When producing aluminum fluoride in accordance with Austrian Patent Specification No. 259 522 and in accordance with the reaction H.sub.2 SiF.sub.6 +2Al(OH).sub.3 .fwdarw.AlF.sub.3 +SiO.sub.2 +4H.sub.2 O, several troublesome waste products are obtained, which in the case of existing production plants are either safely dumped or rendered harmless, normally at significant costs.
The reactions taking place when manufacturing aluminum fluoride are given below, the waste products being underlined. EQU H.sub.2 SiF.sub.6 +2Al(OH).sub.3 .fwdarw.2.alpha.-AlF.sub.3 ( )+SiO.sub.2 1. EQU .alpha.-AlF.sub.3 ( ).fwdarw..beta.-AlF.sub.3 .multidot.3H.sub.2 O+mother liquor 2. EQU .beta.-AlF.sub.3 .multidot.3H.sub.2 O.fwdarw.AlF.sub.3 +3H.sub.2 =+scubber solution 3.
As will be seen from the reaction formulae there is obtained a silica waste which contains aluminum and fluorine impurities. In addition, there is obtained from the crystalization of AlF.sub.3 .multidot.3H.sub.2 O a mother liquor, and from the gas purification process, acid washing liquors--scrubber solution--that contain aluminum, fluorine and silicon. These waste solutions are obtained in relatively large quantities and need to be subjected to expensive treatment processes, before they can be dumped. The mother liquor and washing liquors must be neutralized with calcium hydroxide to a pH of about 8-9, at which the impurities, mainly aluminum, fluorine and silicon, and also minor quantities of phosphorous, iron, nickel and chromium, precipitate out. This precipitation is then isolated from the aqueous slurry, by centrifugation for example. The resultant sludge can be dumped, although because of the consistency of the sludge it is normally necessary to first mix the sludge with a dry carrier material, normally at significant costs.
When mother liquor and gas washing liquors derived from the manufacture of aluminum fluoride from aluminum hydroxide and hexafluorosilicic acid are mixed and neutralized with calcium hydroxide to pH 8-10, preferably 8-9, the impurities are precipitated essentially in the form of aluminum fluorine and silicon, which subsequent to separation, preferably by centrifugation, are recovered in the form of sludge, whereafter the filtrate is passed to a recipient. Subsequent to drying the sludge at a temperature of about 110.degree. C., for example in a spray dryer, flash dryer or rotating drum dryer, there is obtained an ultra-fine dust which contains 8-12% aluminum, 23-28% fluorine, 22-30% calcium, 6-15% silica and bound water corresponding to about 20% calcination loss.
At least 90% of the ultrafine dust has a particle size of between 1 and 45 .mu.m and a specific surface area of about 2100 m.sup.2 /kg according to BLAINE.
None of the methods recommended hitherto for working-up mother liquor and gas-washing liquors has been applied industrially.
SE-B-366 723 relates to a method for recovering fluorine and aluminum from diluted aqueous solutions, preferably mother liquor obtained when crystalizing aluminum fluoride, the mother liquor being treated with ammonia or ammonium carbonate to provide a chiolite-analogue ammonium compound (NH.sub.4).sub.5 Al.sub.3 F.sub.14.
SE-B-No. 7502835-7 relates to the manufacture of a concentrated synthetic flux based on calcium chloride, wherein residual fluorine quantities from waste solutions subsequent to the manufacture of aluminum fluoride are precipitated with the aid of a calcium compound.
SE-A-No. 8100218-0 relates to a method for recovering usable products from the waste products described in the introduction, in which method the contaminated silica waste is dissolved in a strongly basic hydroxide. The resultant first solution is mixed with a second solution obtained by dissolving aluminum hydroxide with a strongly basic hydroxide, and with waste mother liquor and gas-washing liquor derived from the manufacture of aluminum fluoride, in proportions such that the pH of the mixture lies between 10 and 14, wherein the silicon content of the waste products concerned are precipitated as a silicoaluminate, which is isolated either for further processing or for direct use, whereafter fluorine from the waste products is recovered from the filtrate, by adding an aluminum compound in an amount sufficient to precipitate all the fluorine content as fluoroaluminate. This is isolated by filtration for further use, whereafter the final filtrate is discharged to a recipient.
SE-A-No. 8200045-6 relates to a method in which an alkali metal salt is added to a mixture of mother liquor and gas-washing liquor, whereafter an alkali metal base is added to adjust the pH to 2.0-5.0 while maintaining the temperature in the mixture above 50.degree. C. The alkali metal fluoroaluminate precipitated from the mixture is extracted by filtration, and there is obtained a final filtrate which contains silica and minor quantities of aluminum and fluorine.
Side-by-side with these more complicated and more expensive methods remains the desire to find a use for mother liquors and gas-washing liquors obtained as waste when manufacturing aluminum fluoride.
It is known that ultrafine silicon dust, obtained as a residue when manufacturing metallic silicon or the alloying substance silicon iron, has found a use as so-called pozzolan in the manufacture of concrete (Sellevoid E. et al: Condensed silica fume in concrete; Institute for Byggningsmateriallaere, NTH, Trondheim 1982 respective Bache, H: Densified cement/ultrafine particle-based materials. Proc. 2:nd Int. Conf. on Superplasticizers in concrete. Ottawa June 10-12 1981). The silica fines comprise spheroids having a particle size of between 0.01-1 .mu.m (about 70%&lt;0.1 .mu.m), i.e. about 100 times smaller than cement particles, and a specific surface area of 20,000 m.sup.2 /kg.
A pozzolan reacts chemically with hydrated lime to form calcium silicate hydrates similar to those formed when hydrating cement and which provide a denser cement gel, i.e. the strength-creating component of concrete. The pozzolan effect of the silicon fines is attributed partly to the fineness of the particles, partly to their spheroidal shape, and partly to their extremely large specific surface area (Sellevold, E: Silika i betong. Virkemate som pozzolan och filler. BALLASTDAGER, Lule .ANG. 13-15 Sept. 1982).